1. Field of this Invention
This invention relates to the production of 6-substituted-4-methyl-pyridones.
2. Prior Art
2-pyrones have been reacted with ammonia or its salts in the presence of amines, especially substituted aminopyridines or imiazoles, to produce the corresponding 2-pyrones--see German OS 2,214,608. It is known to acylate .alpha.,.beta.-unsaturated carboxylic acid esters, for example, .beta.,.beta.-dimethyl-acrylic acid-ethyl ester, into .delta.-oxo-.beta.-methyl-.DELTA..sup..alpha.,.beta. -hexonic acid ethyl ester in the presence of AlCl.sub.3 with acetyl chloride (see I. Alkonyi, Report 98, 3099 (1965). The cyclization of the unsaturated .delta.-ketoacid ester with acidic or basic catalysts into the corresponding pyrones are known from Belgian Pat. No. 643,891. Furthermore, it is known from South African Pat. No. 6,906,039 to react unsaturated .delta.-keto ester with amines to produce 2-pyridones or from English Pat. No. 851,033 to react pyrones with amines into analogous pyridones. Such processes, however, are in most instances uneconomical for the production of 2-pyridones because of the expensive educts or because the method of processing is too complex. Other processes, in turn, may not simply be transferred to and modified for the production of 2 -pyridones.
According to the Lewis definition, an acid is a substance that can take up an electron pair to form a covalent bond. Thus an acid is an electron pair acceptor. See pages 23 and 24 of Morrison, Robert T., et al., "Organic Chemistry", Allyn and Bacon, Inc., Boston, (1959). A proton is an acid because it is deficient in electrons, and needs an electron pair to complete its valence shell. Hydroxide ion, ammonia, and water are bases because they contain electron pairs available for sharing. In boron trifluoride, BF.sub.3, boron has only six electrons in its outer shell and hence tends to accept another pair to complete its octet. Boron trifluoride is an acid and combines with such bases as ammonia or ethyl ether. Aluminum chloride, AlCl.sub.3, is an acid, and for the same reason. In stannic chloride, SnCl.sub.4, tin has a complete octet but can accept additional pairs of electrons (e.g., in SnCl.sub.6.sup.--) and hence it is an acid, too. In speaking about this kind of acid, the expression Lewis acid, or sometimes acid in the Lewis sense, is used. To be acidic in the Lewis sense, a molecule must be electron-deficient; in particular, one looks for an atom bearing only a sextet of electrons.
Robertson, G. Ross, et al., "Laboratory Practice of Organic Chemistry", 4th Ed., The Macmillan Company, New York, (1962), page 272, says that Lewis acids are catalysts. When an alkyl halide is treated with anhydrous aluminum chloride, the strongly electrophilic aluminum compound attracts a fourth electron-rich halogen atom to complete a group of four electron pairs, to form AlCl.sub.4.sup.-. Such possible reaction becomes feasible if the positive alkyl radical, about to be left behind, simultaneously has the opportunity to attack another electron-rich compound such as benzene. In the present example, benzyl benzene, or diphenylmethane is produced. Although the reagent benzyl chloride contains a benzene ring, its reactive zone is outside the ring and it qualifies here as an "alkyl" derivative. Since aluminum chloride (the Lewis acid) is regenerated, it may be termed a catalyst. An acid anhydride may be substitued for the alkyl halide. By catalytic action of the aluminum chloride, one of the carbonyl groups of the anhydride is set free, ready to attack benzene as the alkyl group did in the previous example. An aromatic keto acid results, such as the .beta.-benzolypropionic acid.
Lewis classed boron chloride as an acid and defined an acid as a substance which can fill the valence shell of one of its atoms with an unshared pair of electrons from another molecule. In favor of this concept is the fact that substances such as boron fluoride, aluminum chloride, zinc chloride or stannic chloride can catalyze the same types of reaction, for example, polymerization of olefins or the formation of ethers, as can a proton. Such compounds often are called Lewis acids. With this definition of an acid, it should be remembered that hydrogen chloride, sulfuric acid, acetic acid, and in fact any of the countless number of substances that from the beginning of chemistry have been called acids, are not acids under the Lewis definition. The acid as defined by Lewis is the bare unsolvated proton, which is practically incapable of existence. Moreover, substance such as cupric ion, which seldom are considered as acids, are acids in the Lewis sense, because they can fill their valence shell with unshared pairs from other molecules, as when cupric ion reacts with ammonia molecules to give the cupricammonia complex ion. Instead of calling all types of compounds capable of accepting a pair of electrons acids, Sidwick in his book on valency published in 1927 called them electron-acceptors, thus leaving the term acid for those compounds capable of transferring a proton to a base. Perhaps a better term than electron-acceptor for reagents, such as, boron fluoride, aluminum chloride, stannic chloride, or zinc chloride that behave like a proton would be protonoid or protonoid reagent. This term would imply properties similiar to those of a proton but would not group these compounds with the substances commonly called acids. See page 235 of Noller, Carl R., "Chemistry of Organic Compounds", W. B. Saunders Company, Philadelphia, (1951).